Mass transfer and distillation columns of various types have been known in the art for many years. Traditionally, enhanced vapor-liquid contact in columns of this type has been effected through the use, for example, of perforated or sieve trays whereby the rising vapor passes through openings, or through bubble caps, in trays on which a pool of the liquid is maintained at a significant depth. By this means, the vapor-liquid contact is controlled and enhanced.
Alternatively, it has been suggested that vapor-liquid contact can also be achieved through the utilization of packing elements in lieu of bubbling the vapor through pools of liquid on trays. Packing elements such as saddles and Raschig rings are well known in the art for such employment. Further, a variety of materials of fabrication for the trays and packing materials have been suggested. Illustrative of these are carbon steel, stainless steel, aluminum alloys, copper alloys, and plastics of various types.
It has also been recognized that certain of these materials are not universally suitable for all types of employment. Thus, for example, carbon steel and plastics generally become embrittled, and thus unsuitable, at cryogenic temperatures. On the other hand, however, costs have militated against the utilization of relatively scarce and expensive materials, such as copper. Aluminum alloys and stainless steel have, however, been widely utilized in the fabrication of trays for cryogenic air separation. In fact, stainless steel has been widely used in oxygen service (both liquid and gaseous) at pressures in excess of 3000 psig. Thus, it would appear that materials such as aluminum and stainless steel would be suitable candidates for fabrication of packing elements for use in cryogenic air separation columns.
We have discovered, however, that the utilization of materials previously found acceptable in cryogenic air separation are not suitable for utilization in the fabrication of packing elements when such packing is to be utilized in cryogenic air separation service. The primary reason for this is that certain materials which are acceptable when utilized in the form and in the manner traditionally employed do, however, present a risk of flammability when employed as packing elements in cryogenic air separation due again, to the form of the material and the conditions prevailing in the vapor liquid contacting apparatus. Thus, for example, stainless steel and aluminum have been utilized to fabricate trays, such as sieve trays, for use in distillation columns in cryogenic air separation service. Such trays, however, generally have a thickness in excess of about one millimeter and generally ranging up to two or three millimeters in thickness. Additionally, these trays usually contain a liquid inventory equivalent to a depth of 30 to 50 or more millimeters. The thickness of the tray alone militates against the propagation of combustion of the material, even in the presence of a relatively high concentration of oxygen, and the presence of the liquid inventory on the tray would act to quench the combustion reaction. As distinguished from this, the material used to fabricate packing elements is relatively thin and thus more susceptible to combustion. Further, the liquid being contacted with the vapor is in the form of a thin film on the surface of the elements, which film is several orders of magnitude thinner than the height of the liquid inventory on a tray.